Digital broadcasting for broadcasting digital contents has started in recent years. Digital contents hardly deteriorate in quality in use. As a result, for the purpose of protecting the copyright, Copy Control Information (CCI), which indicates whether a copy of the content is permitted to be made, and in case of being permitted, the number of copies permitted to be made, is attached to the digital contents. In many cases, the CCI indicates that copying is permitted only once (Copy Once). When a content containing CCI indicating Copy Once is recorded on a recording medium, the CCI is rewritten to indicate that copying thereafter is not permitted (No More Copy). A content containing CCI indicating No More Copy cannot be copied, but can be moved to another medium.
Operation in an unauthorized manner can be performed even on a content containing the CCI. For example, a content recorded on a hard disk recorder (hereinafter referred to as HD recorder) might be operated in an unauthorized manner using a personal computer while the HD recorder is powered off. There is known a technology for preventing such an unauthorized copying by preliminarily calculating unauthorized operation detection information by substituting a content recorded on the HDD into a one-way function, then after the HD recorder is powered on, generating verification information by substituting the content recorded on the HDD into the one-way function, and detecting an unauthorized use of the content by comparing the generated verification information with the stored unauthorized operation detection information.
There is also known a technology intended to improve the convenience of the user by storing a content temporarily, and deleting the content after a predetermined period passes or after the content is played back, thereby realizing a “shift playback” of a content containing CCI indicating that copying is prohibited.
On the other hand, users can record digital broadcast contents on the HD recorder without minding the storage capacity, and can view the recorded contents with simple operations, since the HDD has a large capacity and a random access is available with the HDD.